GB2209029A - Production of intermetallic particles - Google Patents

Description

22090a

PRODUCTION OF INTERMETALLIC PARTICLES AND PRODUCTS INCLUDING SUCH Background of the invention

This invention relates to forming intermetallic compound particles in a molten media such as molten metal. The particles can be left in the media as alloying or reinforcing material or can be sephrated.

Many different methods have been employed to add alloying elements to molten metals. Conventional methods typically add the elements directly to the melt in the form of a lump, a bar or the like. In some cases, they are added directly to molten metal being tapped into a ladle, and in other cases, they may be placed in the ladle prior to tapping.

Another method for adding alloying elements to molten metals, particularly molten steel, is disclosed in U.S. Patent 3,768,999 to Ohkubo et al. In Ohkubo, alloying is accoffiplished by feeding a wire rod into the molten metal. The rod is coated with additives for the molten metal and an organic binder which decomposes into gaseous products in the molten metal. The generated gas stirs the molten metal and thus uniformly incorporates the added components throughout the molten metal.

U.S. Patent 3,729,309 to Kawawa also discloses a method for adding alloying elements in the form of a wire rod U.S.: 090,825 Ecker to molten metals. The rod has a controlled size and is added to a molten metal bath by inserting it at a controlled speed, so as to produce a refined and purified metal alloy.

The above methods of adding alloying elements to molten metal work fairly well with alloying elements which dissolve, melt or disperse easily in the molten metal. However, such methods do not work as well with elements having limited liquid solubility such as Pb, Bi and Sn. or readily high oxidized elements such as Mg and Zn.

U.S. Patent 3,947,265 to Guzowski et al proposes a solution to the problem of adding such "hard-to-alloy" materials to molten metal. The process employs a high current arc which is formed between the molten base metal and the alloying addition. The alloying addition is passed through the arc where it is melted and converted into a spray of finely diviQed superheated molten particles. In such a condition, the particles are able to rapidly dissolve in the molten metal upon contact therewith. While the Guzowski concept of alloying is certainly an interesting one, a need still exists for a process capable of providing improved results.

4 1 In accordance with the present invention, there is provided a process for forming one or more intermetallic compounds comprising the steps:

(a) providing a chamber having a discharge opening positioned within a molten media; (b) introducing into said chamber a gas comprising an ionizable gas under sufficient pressure to provide an interior molten media surface substantially at said chamber's discharge region; (c) providing a plasma within said chamber, said plasma substantially extending at least from said interior molten media surface to a site within said chamber and spaced from said interior media surface; and (d) providing to said site within said chamber material comprising one or more constituents reactable with each other or with one or more constituents in said media, or both, to produce one or more intermetallic compounds and converting said material into superheated spray substantially within said plasma and carried toward said interior molten media surface.

Brief Description of the Drawings

Figure 1 illustrates an embodiment of the present invention.

Figure l."

Figure 3 is a graph Plotting alloy dissolution rate in pounds per minute versus spark cup immersion depth.

Figure 4 is a graph illustrating the relationship of actual recovery in percentages versus immersion depth in inches.

Figure 2 illustrates the spark cup depicted in 1 Figure 1 illustrates the addition of a wire 10 of material into a bath or melt 12 of molten media in a flow-through furnace 14. The surface of melt 12 is referred to herein as exposed or exterior surface 16. Wire 10 is being fed by a feeder 18 which passes it through a triplex feed cable 20 into a spark cup 22, the spark cup being partially immersed in melt 12. In spark cup 22, alloy wire 10 is converted into a spray 24 of superheated material by passing it through a plasma arc discharge (not numbered). The plasma arc discharge is established between a submerged surface 26 of the molten metal which is maintained within an open end 28 (see Fig. 2) of spark cup 22 and a free end 30 of wire 10. The arc discharge is shielded with a shielding gas 32, preferably argon, which is provided via feed cable 20 by an arc shielding gas source 34. In addition to providing a shielding atmosphere for the arc in the spark cup, the shielding gas sourde 34 pressurizes the spark cup at a pressure which is sufficient to prevent molten metal from entering open end 28 of the spark cup. Such pressurization also facilitates maintenance of the aforementioned submerged surface at a certain predetermined depth belbw exposed surface 16 (more on this, infra). Returning to Figure 1, it will be seen that the arc discharge is powered by a constant current power supply source 36 (more on this, infra). Melt 12 serves as an anode with wire 10 serving as a'consumable electrode. The electrical circuit leading back to current source 36 is completed by a return wire 38 which is attached to a rod 40 immersed in melt 12. The superheated spray produced by the arc discharge is directed or projected by the supply of shielding gas onto submerged surface 26 where the material rapidly dissolves and disperses in melt 12. The gas is preferably supplied at a flow rate that maximizes the projection of the spray into the melt. An impeller 42 or agitating means is also provided to further enhance dispersion of the alloy material throughout the melt. Spray 24 can be maintained as long as is desired by continually advancing or feeding the wire into the spark cup. Feeder 18 can also be controlled to maintain or vary the rate at which wire 10 is fed into the spark cup.

The material can be provided in wire form, as described above, or in the form of rod, tube, strip or in powdered form wherein the powders are encased in a hollow tube made from a suitable metal which has been swaged or otherwise worked to reduce its diameter and compact the powdered material in the tube. The only real limitation is that the material should have a form which permits it to be fed into the feed cable in a seal-tight fashion, thereby enabling the pressurized atmosphere in the spark cup to be maintained. If the pressurized atmosphere in the spark cup is not maintained, molten metal will, quite obviously, ent er the spark cup through its open end 28, thereby raising submerged surface 26 to a depth above its predetermined depth. Such raising of submerged surface 26 will result in lower dissolution and dispersion rates. (The impdrtance of maintaining submerged surface 26 at its predetermined depth will be discussed in more detail, infra.) While no means for sealing the wire is depicted in Figure 1, those skilled in the art will be aware of numerous means having the capability of providing an effective seal. Such means could include elastomer and pneumatic seals. In addition, feeder 18 is preferably a consistent feed rate tractor drive.

Constant current source 36 is preferably of the type which maintains a relatively constant current regardless of voltage fluctuations. The arc produced thereby has self-stabilizing characteristics and is relatively insensitive to changes in arc length which might be caused by fluctuations in the submerged molten metal depth. It may also be desirable in certain situations to further enhance arc stability by seeding the plasma discharge with certain additives, such as alkali metals which are known to promote arc stability. Arc stability can also be enhanced by using various fluxes known to those skilled in the relevant art.

As mentioned in U.S. Patent 3,947,265 to Guzowski, may be desirable to add a high frequency, high voltage component to the arc which is particularly useful if AC current is used. -This apparently reduces the tendency of the arc to extinguish every time the voltage passes through zero, increases the stability of the arc and makes initiation of the arc less difficult.

The current supplied by power source 36 exceed the globular/spray transition current density of the introduced material. As used herein, the globular/spray transition current density defines the boundary line separating the two different types of metal transfer that are capable of occurring in the plasma arc discharge. (As pointed out by Guzowski in U.S. Patent 3,947,265, this transition point can vary with such factors as alloy type, wire size and wire speed.) In cases with current densities below the transition point, material being transferred through the arc detaches into large drops which dissolve and disperse slowly in the molten metal media. At current densities above the transition point, the transfer mechanism changes causing the material to convert a fine spray of superheated material. In this condition, the material rapidly dissolves and disperses in the molten media upon contact with submerged surface 26.

Shielding gas 32 carrying or projecting spray 24 into the melt also typically enters the melt. This, however, should not introduce or cause any melt contamination since such gas simply escapes from the melt by bubbling through the melt to exterior surface 16. As previously mentioned, the preferred shielding gas is argon; however, other shielding gases, such as helium, carbon monoxide and carbon dioxide, may also be used in appropriate situations.

The spark cup is preferably cylindrically shaped. Such a shape provides a relatively high spark cup surface area to volume ratio which facilitates conductive heat transfer from the spark cup to the melt. It is important to facilitate such 1 1 heat transfer to prevent the spark cup from overheating. Moreover, those skilled in the relevant art will appreciate that such heat transfer to the melt is advantageous in that it provides a convenient way of adding heat to the melt, thereby r ducing furnace fuel needs. Conventional processes such as that disclosed in Guzowski et al U.S. Patent No. 3,947, 265 do not add much, if any, heat to their respective melts. For example, most of the heat generated during melting of the alloy material in Guzowski et al is lost to the atmosphere since the superheated spray is formed entirely above the melt surface.

The spark cup's cylindrical shape also enhances projection of the shielding gas carrying the superheated spray into the melt. Such projection is important in that it enhances dissolution and dispersion of the alloying material into the melt. While a cylindrical shape is preferred, other shapes, such as an inverted frustoconical shape, which provide enhanced projection and heat transfer are considered to be within the purview of the present invention.

The spark cup should preferably be made from material having the following chaiacteristics:

1. High radiation heat transfer so as to maximize the transfer of radiation heat from the arc discharge to the melt, thereby reducing the possibility of overheating in the spark cup.

2. High resistance to thermal and mechanical shock.

3. High thermal and chemical stability in the melt. Borosilicate, alumina, mullite and silica are some materials known to possess the desired characteristics.

Spark cup is immersed and submerged surface 26 in the open end of the spark Cup is maintained at its predetermined depth below exposed surface 16. Such depth will be referred to hereinafter as the predetermined immersion depth. It has been found that a difference of one or two inches in the immersion depth can have a significant impact upon the rate at which the added material dissolves and disperses in the molten media. Figures 3 and 4 set-forth test data from experiments conducted to determine the effects of immersion depth upon dissolution and dispersion. Figure 3 sets forth data respecting dissolution rate in pounds per minute versus immersion depth, and Figure 4 shows actual recovery in percentages versus immersion depth. The goal of the experiments was to add 0.5% lead to a substantially lead-free body of molten aluminum. The experiments were conducted with a setup similar to that disclosed in Figure 1 except that a constant voltage supply source was used instead of thb priferred constant current supply source. The flow-through furnace used in the experiments contained approximately 1000 pounds of aluminum. The bath of molten aluminum in the furnace had a depth of approximately 30 inches with a diameter of approximately 23 inches. One-eighth inch diameter lead wire was fed into a borosilicate spark cup at a 1 1 feed rate of about 30 inches per minute via a triplex-feed cable. The spark cup was cylindrically shaped and had a lower opening similar to that described in Figure 1 with a diameter of approximately five centimeters. The spark cup's length to diameter ratio was approximately 6 to 1. Argon shielding gas was fed into the spark cup via the feed cable at a flow rate of about 10 standard ft31hr. A plasma arc discharge was established in the spark cup between the free end of the lead wire and the submerged molten metal surface at a voltage of about 35 volts and a current of about 125 amperes, which translates into a current density of about 10,000 amp/in2. As such, the free end of the wire melted and converted into an axial spray of superheated alloy material upon entering the arc discharge. The spray was directed onto the tubmerged melt surface by the shielding gas. After adding an appropriate amount of lead wire to the bath of molten aluminum, the alloyed molten aluminum was continuously cast into several ingots having dimensions of 6 in. x 6 in. x 36 in.

From Figure 3, those skilled in the art will appreciate that a dramatic increase in lead's dissolution rate (that is, the rate at which lead dissolved into the molten medii) regulted when the spark cup immersion depth was increased from five to six inches. 'It will be noted that further increases in immersion depth did not seem to have much of an effect upon the dissolution rate. Similarly, in Figure 4, it can be seen that actual recovery in percentages (i.e., the percentage of alloying material added which actually t -12dissolved in the molten media) increased dramatically when the immersion depth was increased from four to six inches. Moreover, further increases in the immersion depth showed further increases in actual recovery; however, not nearly as dramatic as those that occurred from four to six inches. Actual recovery was measured by optical emission spectroscopy.

The invention iS concerned with forming intermetallic particles such as TiA13, Ni3A1 and other particles. The particles may be geometrically close packed intermetallics (GCP) or they may be topographically close packed (TCP) particles.

Particles such as titanium aluminide, nickel aluminide and other intermetallic particles are often used as reinforcing media to strengthen or otherwise enhance a metal matrix such as aluminum. The benefits of such are generally recognizea but the cost of intermetallic particles is generally considered quite high and those cost factors can impede the usefulness of metal matrix composites so reinforced by intermetallic compounds in cost-sensitive applications. The present invention facilitates providing such intermetallic compounds at a reduced cost in comparison with such other 3 t -13methods commonly employed in producing those compounds such as vapor or chemical vapor disposition or reacting in a high temperature bath of substantial volume and solidifying. For instance, the production of nickel aluminide typically utilizes specialized induction heated furnaces capable of holding at 28000F, an expensive operation at best whereas the present invention produces Ni3A1 in a much less costly operation.

In practicing the invention, referring to Figure 1, the molten media 12 may contain one or more constituents of said intermetallic compound and the material 10 fed to the chamber 22 can comprise another constituent of said intermetallic compound. For instance titanium aluminide (TiA13) can be formed by adding titanium rod 10 to chamber 22 in Fig. 1 immersed in a bath 12 of molten aluminum. The metallic titanium rod 10 is converted into a superheated spray, said spray including titanium vapor spray, and the introduced titanium reacts in the molten aluminum to form titanium aluminide (TiA13) in situ in the molten aluminum. It is believed that the extremely high plasma energy in the chamber 22 vaporizes substantial portions if not all of the titanium such that it forms a vapor spray or other suitable fine spray condition-that is carried into the moltek metal by the gas exiting chamber 22 into the molten media. Thus gas transport may be employed as a mechanism to enhance distribution of the ultrafine titanium spray into the molten aluminum to form titanium aluminide in situ in the molten aluminum. This provides an extremely fine and uniform distribution of the 1 -14titanium in the molten aluminum for prompt reaction therewith to form a uniform dispersion of fine titanium aluminide particles in the molten aluminum. It is worth noting in this connection that plasmas typically reach core temperatures of approximately 50,000OF whereas titanium vaporizes at around 5,9300F. It is to be appreciated that many metals commonly used in alloying vaporize below 10,OOOOF (Cu 4,6800; Ti 5, 930; Zr 7,8200; Nb 8,9060; W 9,9860; Ni 5,1350; Mn 3,7040; A1 4,4400; Cd 2,7150; Cr 4,7900; Co 5,2150; Pb 3,1800; Li 2,4260; Mg 2,0400; Mo 8,7200; Ta 9,8000; Zn 1,6650; Zr 7,9100; Fe 5,2250; all degrees F and approximate). Accordingly, 50,OOOOF typical plasma core temperatures are sufficient to vaporize or finely distribute such metals if this energy is properly harnessed and utilized as in the present invention. Part of this utilization is achieved by using a relatively small chamber such as depicted in Fig. 1 and a carrier gas which can be the ionizable gas used in forming the plasma, and preferably is said ionizable gas, so as to utilize the marked benefits of gas transport mechanisms to provide for greatly enhanced contact surfaces or condensation sites which, in the case of Ni in aluminum, provide for great multiplicities of reacti6n sites to produce multiplicities of nickel aluminide particles.

When referring to a constituent, such is intended to include an element or metal or ingredient or substance that goes into forming the intermetallic compound. The constituent is typically a metal such as nickel or titanium but could Z 1 1 1 1 -is- possibly be a compound including a fluid or gas compound. For instance, an aluminum rod could be introduced to.the chamber 22 which has its discharge immersed in molten aluminum. Helium is provided as an ionizable gas and a plasma is formed as described herein. Methane, acetylene or other gaseous carbon source or even a solid carbon source is also provided to the plasma to form aluminum carbide, A14C3. When referring to a constituent of an intermetallic compound or in referring to a constituent that is reactive to form said compound, what is intended herein is that the constituent be suited to such reaction and so suited in practicing the invention. For instance, titanium is not normally considered to be very reactive in molten aluminum. But it is known that titanium is capable of being reacted with aluminum under certain conditions such as those normally used in producing titanium aluminide. However, as just stated, merely inserting a rod of titanium into a molten aluminum bath is not going to effect this purpose in an efficient manner at normal aluminum processing temperatures of below 1500OF and certainly not in any manner insofar as producing useful titanium aluminide particles. Nonetheless, in practicing the invention where the titanium is introduced. into the plasma arc in accordance herewith and introduced and converted into a vapor or ultrafine spray, it is reactable in molten aluminum.

The process of the invention is considered useful to -produce the number of intermetallic compounds employing one constituent in the form of a solid metal rod being provided as -16material 10 in chamber 22 shown in Fig. 1 and the other being provided as a molten metal bath into which the chamber 22 is immersed. The following table sets forth a listing of applications to which the present invention is considered useful:

Table I

Intermetallic Molten Compound -- Media, MetC Rod 10 Ni3A1 Al Ni FeA13 Al Fe FeA13 Fe Al CrNi3 Ni Cr MnNi3 Ni Mn W2Fe3 Fe W CeFe5 Fe Ce VA13 Al v In practicing the invention, the yield of intermetallic compound particles is increased by increasing the addition rate of material 10 into the chamber 22 and the plasma. The volume fraction of intermetallic particles can be quite high, 10% or 15% or 20% or more, such as fractions of 25 or 30% or more. Another consideration in increasing the yield is to keep the media moving past the chamber exit region so as to provide fresh media for reaction and remove intermetallic compounds formed at the discharge region. The size of the particles can be increased, if desired, by holding such at saturation conditions at the reaction temperature so as to increase reaction residence time. Intermetallic compound formation normally requires elevated temperature. For TiA13 the reaction temperature is about 6650C (about 1230OF). The i g! enormous thermal energy of the plasma is helpful in heating the media or keeping it at a sufficient temperature to favor formation and growth of intermetallic compound particles.

After the intermetallic particles are formed, they can be separately recovered and this can be effected by filtration or centrifuging or possibly by dissolving in a suitable agent or by combinations of operations. Filtration or centrifuging recover the particles as an occluded mass. For instance, in making nickel aluminide in an aluminum matrix the particles are recovered with occluded aluminum. The recovered mass may be useful as is or further processing or separation steps such as dissolution of the occluded aluminum may be desired. For instance, in making titanium aluminide or nickel aluminide, sodium hydroxide is a useful means to dissolve excess aluminum and yield the particles since the sodium hydroxide will dissolve aluminum but not the intermetallic compound. In some cases, the particles are not separated but left in a metal media to provide a metal matrix reinforced with the particles.

Another embodiment of the invention uses feeding two or more constituents of the desired intermetallic compound to the chambdr 22 for reaction there or in the media or both. For instance, a composite rod of Ti and Al could be fed to a chamber 22 in a magnesium bath.

In an example of the invention, an aluminum bath at 1320OF was provided at 960 pounds per hour. A small chamber 22 was fed with 142 pounds/hour of 1/8-inch diameter nickel wire -18and helium gas. Current at 2300 amps (about 190,000 watts) was provided to the rod. A plasma is formed in the chamber 22 as a result of the foregoing and this resulted in producing about 165 pounds/hour of Ni3A1 in the aluminum which was heated to a temperature exceeding 2400F by the process. The heating of the metal bath or media by the process of the invention is quite significant in producing intermetallic compounds and is an important benefit of the invention since formation and growth of intermetallic compound particles can be favored by relatively high temperatures. When the melt solidified approximately 17% of the mass was Ni3A1 particles.

An embodiment of the invention utilizes a solid body of aluminum or other material in the furnace 14. The chamber 22 is initially positioned above or substantially at the upper surface 16 of said material and an arc is initialed and a plasma generated between rod or wire 10 and the aforesaid surface 16. This results in forming a pool of molten aluminum or other material and the chamber 22, lowered therein, functions with respect to that pool substantially in the general manner described hereinabove. Much the same applies to a particulate or lump charge in furnace 14 except that initially-the chamber 22 can be initially positioned with its disbharge below or within the upper level 16 of the body. These arrangements offer potential in using the plasma energy to help in melting in addition to alloying.

While the invention has been described in terms of preferred embodiments, the claims appended hereto are intended to encompass all embodiments which fall within the spirit of the invention.

p 1 C L-A I M S 1. A process for forming one or more intermetallic compounds comprising the steps:

(a) providing a chamber having a discharge opening positioned within a molten media; (b) introducing into said chamber a gas comprising an ionizable gas under sufficient pressure to provide an interior molten media surface substantially at said chamber's discharge region; (c) providing a plasma within said chamber. said plasma substantially extending at least from said interior molten media surface to a site within said chamber and spaced from said interior media surface; and (d) providing to said site within said chamber material comprising one or more constituents reactable with each other or with one or more constituents in said media, or both, to produce one or more intermetallic compounds and converting said material into superheated spray substantially within said plasma and carried toward said interior molten media surface.

2. A process for forming one or more intermetallic compounds comprising the steps:

(a) providing a molten media containing one or more constituents of said compound; (b) providing a chamber having an open discharge positioned within a molten media; (c) introducing into said chamber a gas comprising an ionizable gas under sufficient pressure to provide an interior molten media surface substantially at said chamber's discharge region; (d) providing a plasma within said chamber, said plasma substantially extending at least from said interior molten media surface to a site within said U.S.: 090.825 ECKER 1 A -20chamber and spaced from said interior media surface; (e) supplying to said site within said chamber a material comprising one or more constituents reactable with said one or more constituents in said molten media to produce one or more intermetallic compounds and converting said material into superheated spray substantially within said plasma and carried toward said interior molten media surface for reaction of one or more constituents of said compound in said media with one or more constituents in said material to form said compound or compounds; and (f) conducting a gas from said chamber into said media to aid entry into said media of substances from said chamber.

3. A process according to claim 1 or 2, wherein substantial heat is transmitted into the molten media from the chamber.

4. A process according to claim 1 or 2, wherein the gas is substantially introduced at a rate to aid entry into the molten media of substances from the chamber.

5. A process according to claim 1 or 2, wherein at least a portion of the material is supplied to the chamber means as an elongate solid.

6. A process according to any one of claims 1 to 5. wherein the length of the chamber along the direction of projection into the molten media exceeds the transverse dimension of the chamber in the media.

7. A process according to any one of the preceding claims, wherein the molten media is moving past the discharge of the chamber.

8. A process according to any one of the preceding claims. wherein the media is agitated to further enhance dispersion within said media.

9. A process according to claim 1. wherein two or more constituents of the intermetallic compound are provided to the site in step (d) thereof.

4 10. A process according to claim 1, wherein one or more constituents of the intermetallic compound are provided in the molten media and one or more constituents of the intermetallic compound are 5 provided to saidsite in step (d) thereof. 11. A process according to claim 10, wherein at least a portion of constituents provided to the site in step (d) is provided as one or more fluids. 12. A process according to claim 2, wherein constituents in the material provided to the site in step (e) react with other constituents in said material.

13. A process according to claim 1, wherein at least a portion of the material is supplied as a fluid.

14. A process according to any one of the preceding claims, wherein particles of the intermetallic compound are separated from at least some of the media.

15. A process according to claim 14, wherein the separated particles are treated to remove other substances therefrom.

16. A process according to any one of claims 1 to 13, wherein the particles remain in the media to provide reinforcement therefor in a product comprising at least a portion of said particles and at least a portion of said media.

17. A process for forming intermtallic cmWunds substantially as hereinbefore described.

18. Interffetallic cmWunds when produced by a process according to any one of claims 1 to 17.

1195E a The Ra-C 66 -1 I-_ E221-1crn Londin WC1R 4TF Purther ccpies inay 're obt.,-ine- T..e Pazen., Of:Ice Sales Branch, St Mary Cray, Orpingter.. Kent BR5 3RD PTInted by Multiplex techniques ltd. St Mazy Cray. Kert Con. 1 V.